From what I gather, and for those that have completed this type of course please chime in, is that since the manuveurs for the commercial checkride and CFI checkride are practically identical, you are essentially taking just a CFI course. Once you are prepared enough to take your CFI checkride, you should also be prepared to take your commercial. Do the commerical a day or two before the CFI. So as long as you are proficient with your flying and your knowledge, you in theory will only have to pay for one course. Now, if your flying and knowledge skills are not up to par, then yes you will not save any money since you will be spending more time getting the rust broke loose.